Early hominid physical evolution

Hindrances against bipedalism evolution are localized in obstetrical constraints, maternal mortality rates, infant birth trauma and unsafe pregnancy. Analysis of infant survival probability shows that a shift to bipedalism could occur as a necessary consequence of the process of body fur reduction, in a balance between such hindrances and safe infant transportation. Fur reduction is proposed to correlate with cooling mechanism in intra-species physical fights. The triggering of a feed-back mechanism connecting reduction of body fur to canine reduction would be responsible for a passage from threat displays to actual physical fights. The proposed scenario for such changes is the transition from uni-male to multi-male social structures among Hominoidea. The implications of the approach adopted are discussed.     

Interdisciplinary approaches to advancing anti-racist pedagogies in ecology,evolution, and conservation biology

Racism permeates ecology, evolution, and conservation biology (EECB). Meaningfully advancing equity, inclusion, and belonging requires an interdisciplinary anti-racist pedagogical approach to educate our community in how racism shaped our field. Here, we apply this framework, highlight disparities and interdisciplinary practices across institutions globally, and emphasize that self-reflection is paramount before implementing anti-racist interventions.     

Interdisciplinary approaches to the human biology of South Amerindians

I briefly review the early attempts to establish interdisciplinary investigations in human biology, emphasizing those involving South American Indians. Also considered are (1) the interest in studying host-parasite relationships and the effects of culture changes in the affected populations, (2) the need for the development of appropriate intra-ethnic standards in the study of growth and other quantitative traits, (3) the similarities and dissimilarities of cultural and biologic rates of change, and (4) the purposes of multivariate comparisons. Despite difficulties arising from human nature itself, integrated approaches may yield significant clues to the understanding of evolutionary processes in South American Indians in particular or in our species as a whole.     

Balance of the head in hominid evolution

In primates it is useful to distinguish three basic types of bipedal posture: (1) agonial, with extended hips and knees as in modern humans, (2) monogonial, with flexed hips but extended knees. and (3) digonial, with flexed hips and knees as in pongids. Early hominids retained an ancestral, forwardly inclined posture of the neck and head. Therefore the body posture of australopithednes must have differed from that in modem man, in which the centre of gravity of the head can be aligned with that of the body, other major centra of gravity, and important axes of rotation in a single frontal plane. It is suggested that in australopithednes the gravitational tilt of the head was counterbalanced by bent hips in association with hyperextended knees (monogonial posture). In australopithecines the increase in brain weight would have counteracted an improvement in the balance of the head. After the neck had assumed a more vertical posture as a consequence of shortening of the face, selection for an improved balance system in the bipedal posture favoured an increase in the weight of the postcondylar portion of the head, accentuated by selection for a posterior shift of the superior nuchal line in order to minimise the force of the nuchal muscles. At this stage the evolutionary increase in brain weight may have been largely a by-product of the process towards perfecting the bipedal posture. When the centre of gravity of the head had first become aligned with that of the body, the conditions of balance of the head had become favourable for a dramatic increase of brain size, as a result of selection for greater learning and storage capacity of the brain.     

Locomotor energetics and hominid evolution

The presence of a bipedal gait in fossil apes is now recognized as the earliest paleontological evidence of the beginnings of the human lineage. Thus, the search for the selective pressure that led to the adoption of bipedal posture and gait is the search for the origins of the human adaptation. One of the most popular candidates for the origin of erect posture is its purported energetic advantage.^1–4 This argument is reevaluated in light of data on the energetic cost of locomotion in mammals and, particularly, data on the effect of bipedalism on cost. I go on to discuss what morphological traces we might expect to see of changes in the locomotor economy of our ancestors once bipedalism became established.     

Genetic aspects in hominid evolution

Genomic comparison between apes and humans have made important contributions to our understanding of human evolution. The modern period of karyological comparisons between humans and other primates began about forty years ago and has been marked by a series of technical revolutions. In the 1960s pioneering genetic and chromosomal comparisons of human and great apes suggested, as had Darwin a century before, that our closest relative were the African apes. Early immunological analyses placed human/apes divergence at about five million year ago. Acceptance of man’s late divergence from the African apes was delayed by the scarcity of paleontological evidence coupled with a fallacious Asiatic origin hypothesis of the hominoids. Chromosome banding techniques in the seventies and high resolution methods in the eighties allowed a detailed comparison of the chromosomes between closely related primates and reinforced the hypothesis of an African origin for humans. It was clearly shown that humans were more closely related to African apes than to the orang-utan. The last decade has seen a vigorous integration of molecular and cytogenetic. This powerful combination promises to be quite fruitful because chromosomes can be compared directly at the DNA level. Fluorescentin situ hybridisation (FISH), chromosome painting, is a colourful technique for establishing chromosomal homology between species. Results obtained by FISH over the last ten years have resolved the cytogenetic problem of the homology between humans, apes, hylobates and Old World monkeys and defined the chromosomal syntenies and major translocations involved in the genome evolution of higher primates.     

Variability selection in hominid evolution

Variability selection (abbreviated as VS) is a process considered to link adaptive change to large degrees of environment variability. Its application to hominid evolution is based, in part, on the pronounced rise in environmental remodeling that took place over the past several million years. The VS hypothesis differs from prior views of hominid evolution, which stress the consistent selective effects associated with specific habitats or directional trends (e.g., woodland, savanna expansion, cooling). According to the VS hypothesis, wide fluctuations over time created a growing disparity in adaptive conditions. Inconsistency in selection eventually caused habitat-specific adaptations to be replaced by structures and behaviors responsive to complex environmental change. Key hominid adaptations, in fact, emerged during times of heightened variability. Early bipedality, encephalized brains, and complex human sociality appear to signify a sequence of VS adaptations—i.e., a ratcheting up of versatility and responsiveness to novel environments experienced over the past 6 million years. The adaptive results of VS cannot be extrapolated from selection within a single environmental shift or relatively stable habitat. If some complex traits indeed require disparities in adaptive setting (and relative fitness) in order to evolve, the VS idea counters the prevailing view that adaptive change necessitates long-term, directional consistency in selection. © 1998 Wiley-Liss, Inc.     

Cranial capacity in hominid evolution

We present an analysis of cranial capacity of 118 hominid crania available from the literature. The crania belong to both the genusAustralopithecus andHomo and provide a clear outline of hominid cranial evolution starting at more than 3 million years ago. Beginning withA. afarensis there is a clear increase in both absolute and relative brain size with every successive time period.H.s. neandertal has an absolutely and relatively smaller brain size (1412cc, E.Q.=5.6) than fossil modernH.s. sapiens (1487cc, E.Q.=5.9). Three evolutionary models of hominid brain evolution were tested: gradualism, punctuated equilibrium, and a mixed model using both gradualism and punctuated equilibrium. Both parametric and non-parametric analyses show a clear trend toward increasing brain size withH. erectus and a possible relationship within archaicH. sapiens. An evolutionary stasis in cranial capacity could not be refuted for all other taxa. Consequently, the mixed model appears to more fully explain hominid cranial capacity evolution. However, taxonomic decisions could directly compromise the possibility of testing the evolutionary mechanisms hypothesized to be operating in hominid brain expansion.     

Metabolic correlates of hominid brain evolution

Large brain sizes in humans have important metabolic consequences as humans expend a relatively larger proportion of their resting energy budget on brain metabolism than other primates or non-primate mammals. The high costs of large human brains are supported, in part, by diets that are relatively rich in energy and other nutrients. Among living primates, the relative proportion of metabolic energy allocated to the brain is positively correlated with dietary quality. Humans fall at the positive end of this relationship, having both a very high quality diet and a large brain size. Greater encephalization also appears to have consequences for aspects of body composition. Comparative primate data indicate that humans are 'under-muscled', having relatively lower levels of skeletal muscle than other primate species of similar size. Conversely, levels of body fatness are relatively high in humans, particularly in infancy. These greater levels of body fatness and reduced levels of muscle mass allow human infants to accommodate the growth of their large brains in two important ways: (1) by having a ready supply of stored energy to 'feed the brain', when intake is limited and (2) by reducing the total energy costs of the rest of the body. Paleontological evidence indicates that the rapid brain evolution observed with the emergence of Homo erectus at approximately 1.8 million years ago was likely associated with important changes in diet and body composition.     

Warfare and hominid brain evolution.

This paper reviews literature on the evolutionary effects of warfare upon the hominid brain. Alexander &; Tinkle (1968) and Bigelow (1969) are found to be the first to propose that warfare was the principle evolutionary pressure that created the novel substance of the human brain, and that it acted at least from the early Pleistocene. These writers are distinguished from Darwin (1871), Keith (1947) and Wilson (1975) who saw warfare influencing the development of the brain only in historical or near-historical times.The warfare hypothesis of Alexander &; Tinkle is found to be an excellent explanation of the evolution of the human brain, but to be unsatisfactory from a biological viewpoint because they do not explain how warfare evolved in the first place, nor do they attempt to account for the apparent absence of warfare as a behavioral adaptation in species other than some eusocial insects.This author underpins the warfare hypothesis, arguing that it evolved as a necessary consequence of the circumstances of early hominids. Proficient tool use gave domination over predators and opened up new food resources, thereby diminishing two population controls. A population explosion resulted and, at critical densities, when starvation threatened, warfare was the genetically most successful behavioral adaptation. Alternative hypotheses are shown to be inadequate. Finally, the author asks why such an important hypothesis has been ignored for almost a decade.     

Comparative primate energetics and hominid evolution

There is currently great interest in developing ecological models for investigating human evolution. Yet little attention has been given to energetics, one of the cornerstones of modern ecosystem ecology. This paper examines the ecological correlates of variation in metabolic requirements among extant primate species, and uses this information to draw inferences about the changes in energy demands over the course of human evolution. Data on body size, resting metabolism, and activity budgets for selected anthropoid species and human hunter-gatherers are used to estimate total energy expenditure (TEE). Analyses indicate that relative energy expenditure levels and day ranges are positively correlated with diet quality; that is, more active species tend to consume more energy-rich diets. Human foragers fall at the positive extremes for modern primates in having high expenditure levels, large ranges, and very high quality diets. During hominid evolution, it appears that TEE increased substantially with the emergence of Homo erectus. This increase is partly attributable to larger body size as well as likely increases in day range and activity level. Assuming similar activity budgets for all early hominid species, estimated TEE for H. erectus is 40–45% greater than for the australopithecines. If, however, it is assumed that the evolution of early Homo was also associated with a shift to a more “human-like” foraging strategy, estimated expenditure levels for H. erectus are 80–85% greater than in the australopithecines. Changing patterns of resource distribution associated with the expansion of African savannas between 2.5 and 1.5 mya may been the impetus for a shift in foraging behavior among early members of the genus Homo. Such ecological changes likely would have made animal foods a more attractive resource. Moreover, greater use of animal foods and the resulting higher quality diet would have been important for supporting the larger day ranges and greater energy requirements that appear to have been associated with the evolution of a human-like hunting and gathering strategy. Am J Phys Anthropol 102:265–281, 1997 © 1997 Wiley-Liss, Inc.     

Multiple dispersals and the environment in hominid evolution

Conclusions Given that we have attempted a very broad sweep through two of the major issues in anthropology (human evolution and environmental change) it would be impossible to have any easy conclusions. In closing I would therefore stress only the major points in their simplest form. First, hominid evolution does not consist of just two dispersals (out of Africa 1 and 2) but of multiple such events. Unraveling these, and the ecological conditions underlying them, should be a major focus for research. Second, if hominid evolution does consist of multiple dispersals, then it follows that there must have been multiple local extinctions. Although we tend to think of human demography in terms of growth, demographic collapse is equally important in evolution. Third, where hominids have had an effect on the environment it has tended to occur in two very specific contexts-the colonisation of new regions and the development of agriculture. And finally, if we are interested in the human evolution and the environment, coevolution rather than anthropogenic change may be the more useful concept.     

Longevity and life history in hominid evolution

Under the assumption that life history in general and longevity in particular play an important part in the study of evolutionary patterns and processes, this paper focuses on predicting longevity changes across hominid evolution and attempts to throw light on the significance of such changes. We also consider some statistical arguments in the analysis of hominid life history patterns. Multiple regression techniques incorporating primate body weight and brain size data are used to predict hominied longevity and the results are compared to those in the literature. Our findings suggest that changes in hominid longevity are more likely to follow brain size than body weight, and that multiple regression techniques may be an appropriate avenue for future studies on life history variation in human evolution.     

Observed and expected variation in hominid evolution

All of the major groups of fossil hominids (australopithecines, pithecanthropines, Neandertals, and early sapiens) were discovered by 1925, and therefore prior to the formulation of the synthetic theory of evolution that revolutionized the concept of the species in systematics. While these fossil finds were being made the framework for their interpretation included several assumptions: (1) that the number of living hominoid species was great, and that intraspecific variation was slight (authoritative sources recognized as many as 14 separate species of chimpanzees and 15 species of gorillas); (2) that the timescale of human evolution was brief (measured in tens or hundreds of thousands of years). As a result of these premises the consensus that hominid evolution was characterized by a large number of sympatric and synchronic species was virtually inevitable.In contrast, recent molecular studies demonstrate that genetic diversity among recent hominoids is so slight that even humans and chimpanzees differ at only about 1% of the loci that have been sampled so far; evidently, very small genetic differences can produce rather great contrasts in morphology. At the same time, geological break-throughs have increased the timescale for human evolution to several million years.It is concluded that morphological differences among fossil hominids, even if very appreciable and complex, do not necessarily reflect a great degree of either genetic or taxonomic diversity. Potential effects of evolutionary change through time should be incorporated into models of hominid evolution as a means of assessing the minimum number of lineages required to account for observed variations among hominid specimens.     

Developmental genetics and early hominid craniodental evolution

Although features of the dentition figure prominently in discussions of early hominid phylogeny, remarkably little is known of the developmental basis of the variations in occlusal morphology and dental proportions that are observed among taxa. Recent experiments on tooth development in mice have identified some of the genes involved in dental patterning and the control of tooth specification. These findings provide valuable new insight into dental evolution and underscore the strong developmental links that exist among the teeth and the jaws and cranium. The latter has important implications for cladistic studies that traditionally consider features of the skull independently from the dentition.     

A sexual selection model for hominid evolution

The following paper develops a sexual selection model for the evolution of bipedal locomotion, canine reduction, brain enlargement, language and higher intelligence. The model involves an expansion of Darwin’s ideas about human evolution based on recent elaborations of sexual selection theory. Modern notions about intrasexual competition and female and male choice and their ecological correlates are summarized along with a new model for the role of sexual selection in speciation. Rapid evolution of bipedal locomotion as a male adaptation for nuptial feeding of females is proposed as a model for ape-hominid divergence through sexual selection; canine reduction is attributed to selection for associated epigamic displays. The analogy with male specialization through sexual selection speciation in hamadryas baboons is noted. Subsequent changes in female reproductive physiology are attributed to female competition for increased male parental investment during the time of early Homo andHomo erectus. The origin of higher intellectual and language abilities inHomo sapiens is attributed to male competition through technology and rule production to control resources and females; intellectual abilities involved in social manipulation are attributed to female competition for male parental investment and maintenance of polyandry. The course of hominid evolution is characterized as involving a trend from a promiscuous mating system toward increasing intensity of adaptations for male control of females, and by increasing intensity of female adaptation to maintain male parental investment while circumventing male control.